A. Field of the Invention
The present invention relates to wear or abrasion resistant windows or composites which include optical thin films and have visible spectral applications and, in particular, to such windows which include carbonaceous films.
As used here in reference to the present invention, the term wear or abrasion resistant window or abrasion resistant glass window, refers to a window which is used in applications such as laser bar code scanners and which is transparent over applicable portions of the visible spectrum. The wear resistant window is a composite which comprises a transparent substrate of material such as glass or silicon oxide and one or more thin film coatings which enhance physical and/or optical properties of the substrate such as durability and transparency for visible radiation. The word "window" is used here to denote the substrate on which the thin film coating(s) are formed and as a shorthand reference to the overall coated substrate composite. Also, "index" is used as shorthand for index of refraction.
B. Description of the Related Technology
1. Non-Fluorine-Containing Wear Resistant Glass Window
Wear resistant windows are used in applications such as laser bar-code scanners in point-of-sale (POS) equipment. In some applications such as in the POS equipment used in groceries and convenience stores, wear resistant windows are subjected to severe mechanical abrasion by moving bottles, cans, and other merchandise. Glass windows which are or include ordinary glass typically are quickly scratched beyond usefulness by short periods of use in such an environments. Although crystalline or polycrystalline aluminum oxide (sapphire) is superior in that it is much harder than glass, sapphire is unsuitable for most commercial applications because it is extremely expensive.
High transparency is essential in laser bar-code scanner windows. Ordinary glass has a refractive index of 1.52 and a transmittance of about 92% for the red laser wavelengths which are of primary interest. Sapphire has an index of refraction of about 1.67 and a transmittance of about 88% over the same range of wavelengths. Coated wear resistant windows such as the above product are slightly absorbing, and are less transparent than uncoated glass or uncoated sapphire. Higher transmission is desirable in some cases in order to consistently meet the transmittance specifications. For example, an exemplary minimum transmittance for laser bar code scanner windows is 78% over the 600-650 nm (nanometers) portion of the visible spectrum. Under continuous use (i.e., subjected to mechanical abrasion), minimum transmittance must be maintained for scanner windows to avoid misreads. Therefore, one must balance transparency and resistance to mechanical abrasion in the selection of materials or coatings, to obtain a fully functional product.
Wear resistant window glass coatings are also described in U.S. Pat. Nos. 5,135,808 and 5,268,217. These patents teach that DLC (diamond-like carbon) will not adhere to sublayers containing fluorine and that the material of the layer immediately adjacent to the DLC coating must be a substantially optically transparent material devoid of alkali metal atoms and fluorine. Previous work (e.g., National Bureau of Standards Spec. Pub. 638 (1981) and Proceedings of the DARPA Workshop on DLC Coatings (1982)) discusses the adhesion of DLC to fluoride materials for primarily infrared applications. The literature also discusses the use of fluorine additions or dopants to enhance the electronic properties of metal oxide films, but does not focus on their optical characteristics, or other material properties such as hardness, adhesion to other materials, or film degradation.
2. Exemplary OCLI Abrasion Resistant Glass Window
Optical Coating Laboratory, Inc. previously developed a coated glass wear resistant window that exhibits durability approximating that of sapphire, and is less costly. The wear resistant window comprises two layers coated onto the exposed surface of a strengthened, i.e., tempered glass window or base. The first layer deposited on the glass is a relatively thick or massive silicon oxynitride (SiO.sub.x N.sub.y) layer prepared by reactive sputtering of silicon in the presence of a gas containing oxygen and nitrogen. The silicon oxynitride layer is approximately five micrometers (50,000 Angstroms) thick. The proportions of oxygen and nitrogen are adjusted during deposition to provide a suitable balance of stress, adhesion, hardness and transparency. The second or top layer is diamond-like carbon (DLC) which is approximately 200 Angstroms (0.02 micrometers) thick. The massive silicon oxynitride "hardcoat" layer imparts a high level of hardness or impact resistance to the window composite, while the DLC overcoating or overlayer contributes low friction or lubricity/lubriciousness.
3. Properties of Abrasion Resistant Glass Window (Window+Hardcoat+DLC Overcoat (FIG. 3))
The family 31 of curves, FIG. 3, depicts the measured spectral transmittance of five non-fluorine-containing windows of the type described above. The data for these windows are the first five entries in Table 1. All windows meet or exceed the minimum acceptable 78% average transmittance for the 600-700 nanometers range. In spite of its small thickness, the DLC layer is quite absorbing and lowers the transmittance of the abrasion resistant window from that of the hardcoat itself. As with the hardcoat layer, the optical performance of the DLC is a consequence of optimized mechanical properties. Deposition process parameter variations that result in increased transparency (lower absorption) diminish the critical mechanical properties of stress, adhesion and friction or lubricity.
The measured values for fifteen production samples (including those from FIG. 3) are listed in Table 1. The data in the last column (average transmittance over the 600-700 nanometers wavelength range) is also shown in the family 41 of data depicted in FIG. 4.
TABLE 1 ______________________________________ Measured Transmittanae for 15 Abrasion Resistant Glass Production Samples Sample Measured Transmittance (percent) number 400 nm 632 nm Avg 600-700 nm ______________________________________ 1 56.78% 78.11% 79.02% 2 57.39 80.52 79.42 3 56.56 79.30 79.30 4 56.01 79.21 79.21 5 56.95 79.17 79.17 6 56.89 79.36 79.28 7 56.83 78.21 79.46 8 56.72 80.10 79.42 9 55.94 76.97 78.98 10 55.54 77.79 79.11 11 57.83 80.54 79.47 12 54.29 80.08 78.64 13 54.58 77.75 78.79 14 55.89 78.46 78.76 15 54.60 76.87 78.63 ______________________________________